The present invention relates to a thin film transistor (TFT) and a method of making the same, and more particularly to a thin film transistor having a vertical-double structure and a method of making the same.
Referring to FIG. 1, there is illustrated the structure of a thin film transistor serving as a driving element, in a conventional liquid display device of active matrix type.
Hereinafter, a method of making the conventional TFT shown in FIG. 1 will be described.
On a glass substrate 1, first, a metal such as Cr, Al or Ta is deposited to form a first metal layer and then the metal is patterned to form a gate having a pattern.
Subsequently, an insulation material such as oxide or nitride is deposited on the whole surface of the glass substrate 1 which includes the gate 2 using a plasma enhanced chemical vapour deposition, as first insulation layer 3.
Thereafter, over the first insulation layer 3 are formed a semiconductor layer serving as an active layer 4 and a semiconductor layer 5 in which an impurity is doped, in this order.
The active layer 4 is then patterned so that it has a desired pattern. The semiconductor layer 5 is used as a layer for ohmic contact with source and drain electrodes which will thereafter be formed.
Subsequently, over the exposed whole surface, a second metal layer is deposited and then patterned so that it has desired patterns, thereby to form a source electrode 6 and a drain electrode 7.
A dry etching process is carried out on the semiconductor layer 5 so as to remove the semiconductor layer 5 doped with an impurity corresponding to a channel portion between the source electrode 6 and the drain electrode 7, and hence to form a TFT.
Subsequently, a protection layer 8 is formed on the exposed whole surface of the glass substrate 1 including the TFT, so as to protect the TFT formed as above mentioned.
In case of applying the conventional TFT formed as hereinbefore described to a driving circuit of a liquid display device of active matrix type, a plurality of TFTs constructs a TFT array, so as to individually drive each pixels of the liquid display device of active matrix type.
On the other hand, the number of total TFTs is increased in the TFT array as the liquid display device of active matrix type becomes more on a large scale and in a high-integration.
Therefore, the size of TFT becomes smaller, and hence characteristic thereof is also deteriorated.
Namely, since channel width is also reduced in accordance with the reduction of size thereof, turn-on current thereof is reduced in proportion to the size.
Therefore, it is impossible to obtain image of high quality through the liquid display device.
The increase of number of total TFTs in the TFT array also causes the production rate of TFT to reduced, upon manufacturing TFTs and also causes the opening rate of each pixels of the liquid display device to be reduced, in accordance with the high integration thereof.